Position sensorless open loop control for motor drives with output filter and transformer

ABSTRACT

A power converter, control apparatus and methods are presented for driving a permanent magnet motor or other load through a sine wave filter and a transformer, in which inverter output current is controlled using a current-frequency relationship to convert a desired frequency or speed value to a current setpoint, and the inverter output current is regulated to the current setpoint using a control algorithm with a bandwidth below the resonant frequency of the sine wave filter.

REFERENCE TO RELATED APPLICATION

This application is a continuation of, and claims priority to and thebenefit of, U.S. patent application Ser. No. 13/868,216, filed on Apr.23, 2013, entitled POSITION SENSORLESS OPEN LOOP CONTROL FOR MOTORDRIVES WITH OUTPUT FILTER AND TRANSFORMER, the entirety of whichapplication is hereby incorporated by reference.

BACKGROUND

Sensorless motor drives have been used in a variety of applications,particularly where providing position and/or speed sensors directly at amotor load is difficult or impractical. A typical sensorless systememploys a voltage-frequency (V/F), alternatively known as Volts perHertz (V/Hz), controller providing a voltage setpoint according to adesired motor speed or frequency, and this form of sensorless controlhas been used primarily with induction motors. In certain applications,however, a step-up transformer is often needed to boost the motor driveoutput voltage. For instance, a transformer may allow a low-voltagedrive to be used to power a medium voltage induction motor, and/or astep-up transformer can be used to reduce I²R losses and allow use of asmaller gauge cable wire for long cable runs between the motor drive andthe driven motor. Certain applications also employ sine wave filters,such as LC filters to suppress reflected wave voltage spikes associatedwith pulse width modulated variable frequency drives. Use ofvoltage-frequency control techniques, however, may lead to problems,particularly where a transformer and/or sine wave filter is connectedbetween the motor drive in the motor load. For example,voltage-frequency control loops often suffer from variations inuncontrolled drive current, even when the voltage command is constant.Also, saturation of the step-up transformer may lead to significantlyincreased drive current, without delivering much power to the motorload. Moreover, voltage-frequency control in combination with a sinewave filter under starting conditions may result in the motor not beingable to start, with large oscillations on the rotor shaft for lowfrequency commands. Furthermore, conventional sensorlessvoltage-frequency drive control has not been largely successful indriving permanent magnet motors when output filters and transformers areemployed. Thus, while sensorless control schemes are advantageous due tolength of cable runs and avoidance of costs associated with providingfeedback directly from the motor, further improvements are needed forsensorless motor drive control, particularly for driving permanentmagnet motors.

SUMMARY

Various aspects of the present disclosure are now summarized tofacilitate a basic understanding of the disclosure, wherein this summaryis not an extensive overview of the disclosure, and is intended neitherto identify certain elements of the disclosure, nor to delineate thescope thereof. Rather, the primary purpose of this summary is to presentvarious concepts of the disclosure in a simplified form prior to themore detailed description that is presented hereinafter. The presentdisclosure provides sensorless position control using current regulationand current-frequency and reduced bandwidth control concepts by whichopen loop power converter control is possible to avoid or mitigate theabove-mentioned shortcomings of traditional voltage-frequency sensorlesscontrol. These techniques and apparatus find particular utility inassociation with sensorless motor drive applications involving sine waveoutput filters and step-up transformers to accommodate long cable runsbetween the drive and a driven motor, including induction motors and/orpermanent magnet motors, such as in submersible pump applications andthe like. Other applications are possible, in which the describedcontrol approaches may be used, including power converter operation toprovide variable frequency AC output to any form of load.

A power conversion system is presented, which includes an inverterproviding AC output power to drive a load, as well as a controller thatregulates the inverter output current(s) in whole or in part accordingto a frequency or speed setpoint value via a control algorithm having abandwidth below a resonant frequency of a filter coupled between theinverter and the load. In certain embodiments, the controller includes acurrent-frequency control component providing a current setpoint valueat least partially according to the frequency or speed setpoint, as wellas a current control regulator component implementing the controlalgorithm to regulate the inverter output current or currents at leastpartially according to the current setpoint value. In certainimplementations, the current control regulator may be aproportional-integral (PI) controller with a control bandwidth below theresonant frequency of the output filter, and the control algorithm canbe implemented to regulate the inverter output currents according to thecurrent setpoint value and one or more feedback signals or valuesrepresenting the inverter output current. Moreover, certainimplementations of the controller include a rate limiter componentoperative to limit a rate of change of the received desired frequency orspeed value to provide a rate-limited frequency or speed setpoint.

A power conversion system control method and computer-readable mediumswith computer-executable instructions are provided in accordance withfurther aspects of the present disclosure, in which a current setpointvalue is determined at least in part according to a frequency or speedsetpoint value, and at least one AC output current feedback signal orvalue of the power conversion system is sampled. The method furtherincludes regulating the output current according to the current setpointvalue and the output current feedback using a control algorithm with abandwidth below a resonant frequency of an output filter. In certainimplementations, the method further includes limiting the rate of changeof a desired frequency or speed value to determine the frequency orspeed setpoint, and may also include determining the current setpointvalue according to a current-frequency relationship with a zero currentvalue corresponding to a zero frequency value. In certainimplementations, for example, the current-frequency relationship may bea curve or parametric equation or lookup table or the like, including afirst portion with increasing current values corresponding to a firstfrequency range from zero to a cutoff frequency, as well as a secondportion having a constant current value, such as a maximum outputcurrent of the inverter, for frequencies above the cutoff frequencyvalue.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure will be setforth in the following detailed description when considered inconjunction with the drawings, in which:

FIG. 1 is a schematic diagram illustrating an exemplary variablefrequency drive type power conversion system providing AC output powerthrough a sine wave filter and a step-up transformer and a cable to adriven permanent magnet motor load for submersible pump applications andthe like, in which the motor drive inverter output stage is controlledusing a current-frequency control component and a reduced bandwidthproportional-integral (PI) control component in accordance with one ormore aspects of the present disclosure;

FIG. 2 is a schematic diagram illustrating further details of theexemplary inverter controller in the system of FIG. 1, including a ratelimiter component, a current-frequency control component, and a reducedbandwidth PI control component in accordance with the disclosure; and

FIG. 3 is a flow diagram illustrating an exemplary method forcontrolling a power conversion system driving a load through a filter inaccordance with further aspects of the present disclosure.

DETAILED DESCRIPTION

Referring now to the figures, several embodiments or implementations arehereinafter described in conjunction with the drawings, wherein likereference numerals are used to refer to like elements throughout, andwherein the various features are not necessarily drawn to scale. Thepresent disclosure provides methods and apparatus for driving anelectric motor or other load through an output filter and optionallythrough an additional transformer, and finds utility in submersible pumpsituations or other applications where an AC load is powered withoutdirect feedback from the driven load. For example, sensorless motordrive applications may be enhanced by use of the disclosed apparatus andmethods, even for permanent magnet motor loads, while mitigating oravoiding unregulated drive output current, transformer saturation, andproblems with motor starting seen in conventional voltage-frequencysensorless motor drive systems. Accordingly, the advantages ofsensorless control can be facilitated, including reduced cost and systemcomplexity, in combination with the use of sine wave filters andtransformers to reduce the cost and size of cabling and to mitigatereflected wave problems, while still achieving enhanced controlcapabilities with respect to motor speed and/or position or other drivenload performance parameters. Moreover, the concepts of the presentdisclosure do not require additional hardware, and thus present alow-cost solution to the above-mentioned problems associated withconventional voltage-frequency sensorless motor control schemes.

FIG. 1 illustrates an exemplary system 2 having an AC power source 4providing three-phase AC input power (e.g., 480 V AC, 50 or 60 Hz) to amotor drive power conversion system 10. The motor drive 10, in turn,provides variable frequency, variable amplitude multiphase AC outputpower through a sine wave filter 16 and a connected transformer 18, andthen through a cable 8 to drive a permanent magnet or induction motorload 6 as shown. In various applications, such as submersible pumps, arelatively lengthy cable 8 can be used, and the transformer 18 may beused in certain implementations as a step-up device to boost the voltageoutputs provided by the motor drive 10 to a higher level to combat I²Rlosses along the length of the cable 8 and reduce the size of the cable8, and/or to allow a relatively low voltage motor drive 10 to operate ahigher voltage motor load 6. As seen in FIG. 1, the motor drive 10includes a rectifier 12, which can be an active (e.g., switching)rectifier or a passive rectifier, full wave, half wave, etc., and whichreceives the AC input power from the source 4 and provides DC power to abus or DC link circuit 13 having a capacitance C. While illustrated as amultiphase rectifier, the concepts of the present disclosure may beemployed in single-phase input drives and power converters. An inverter14 receives DC power from the bus circuit 13 and includes switchingdevices S1, S2, S3, S4, S5 and S6 operated according to inverterswitching control signals 22 provided by a controller 20 in order toconvert the DC power to AC output currents IA, IB and IC for driving themotor load 6. In the illustrated embodiment, the inverter 14 provides athree-phase output, but other multiphase and single-phase outputimplementations are possible within the scope of the present disclosure.Any suitable inverter switching devices S1-S6 may be used, includingwithout limitation insulated gate bipolar transistors (IGBTs), siliconcontrolled rectifiers (SCRs), gate turn-off thyristors (GTOs),integrated gate commutated thyristors (IGCTs), etc.

The motor drive 10 also includes a controller 20 providing the inverterswitching control signals 22 to the inverter switches S1-S6. Thecontroller 20 and the elements and components thereof (e.g., furthershown in FIG. 2 below) can include suitable logic or processor-basedcircuitry, and may also include signal level amplification and/or drivercircuitry (not shown) to provide suitable drive voltage and/or currentlevels sufficient to selectively actuate the switching devices S1-S6,for instance, such as comparators, carrier wave generators or digitallogic/processor elements and signal drivers. Moreover, the controller 20can provide the switching control signals 22 according to any suitablepulse width modulation technique, including without limitation vectormodulation (SVM) carrier-based pulse width modulation, selectiveharmonic elimination (SHE), etc.

The system 2 of FIG. 1 also includes a sine wave or output filter 16, inone example, a three-phase LC filter having a series filter inductor LFin each output line, as well as a corresponding filter capacitor CFcoupled between the corresponding phase line and a common connectionpoint. Other output filter topologies may be used, such as LCL filters,CLC filters, etc. with one or more series elements and further filterelements (e.g., filter capacitors CF) connected in any suitable delta orY configuration. In addition, as shown in FIG. 1, a transformer 18 isprovided between the filter 16 and the motor cable 8. In the illustratedexample, the transformer 18 has a three phase delta-connected primary aswell as a Y-connected secondary, although any suitable transformerprimary and/or secondary winding configuration or topology may be used.Moreover, the transformer 18 may, but need not, be a step-uptransformer. In certain applications, a step-up transformer 18 isadvantageous, for example, to allow a low-voltage drive 10 to power amedium or high voltage motor 6, or to allow use of a medium-voltagedrive 10 to power a high-voltage motor 6. Also or in combination, astep-up transformer 18 may be useful to allow a reduction in the currentlevels carried by the cable 8, thereby facilitating use of smallerdiameter cable wires and a corresponding reduction in I²R losses in thecable 8. The cable 8, moreover, can be of any suitable construction forinterfacing the motor drive output, the sine wave filter 16, and thetransformer 18 with the leads of the motor 6.

The motor drive 10 and the controller 20 thereof, operate in sensorlessfashion to control one or more operating parameters of the driven motorload 6. For example, the controller 20 provides the inverter switchingcontrol signals 22 in order to control position and/or speed and/ortorque of the motor 6 without directly sensing any of these controlledparameters. In the illustrated implementation, for instance, currentsensors 27 are provided at the output of the inverter 14 to providefeedback signals or values 28 to the controller 20 which represent theinverter output currents IA, IB and IC, and/or from which the values ofthese output currents can be computed, derived or otherwise estimated.Any suitable current sensing devices 27 can be used to generate thesignals and/or values 28, and may provide analog signals 28 and/or thesensors 27 may be smart sensors providing digital values 28 representingthe output currents IA, IB and IC provided by the inverter 14.

The controller 20 uses the feedback signals or values 28 as well as oneor more desired operating parameters 21 to perform regulation of theoutput currents IA, IB and IC in a localized closed-loop fashion.Overall, however, the control technique implemented by the controller 20is essentially sensorless or open-loop with respect to the actualoperating condition of the driven motor load 6, as there are no directfeedback signals obtained from the motor 6 itself. In the example ofFIG. 1, for instance, the controller 20 receives a desired frequency ormotor speed value f* 21 from a supervisory control system component (notshown), which may be a distributed control system element, auser-adjustable knob, local user interface, etc. The controller 20,moreover, includes a current-frequency control component 24 as well as areduced bandwidth proportional-integral (PI) controller 26 as describedfurther below. In operation, the control components 24 and 26 are usedto regulate the inverter output currents IA, IB and IC via generation ofthe inverter switching control signals 22 according to the desired speedor frequency signal or value 21 and the feedback signals or values 28.

Referring also to FIG. 2, one embodiment of the controller 20 isillustrated, which may optionally include a rate limiter 30, as well asthe current-frequency (I-F) control component 24 and the PI controllerelement 26 in a forward control loop path. If included, the rate limitercomponent 30 receives the desired frequency or speed value 21 and limitsthe rate of change thereof to provide a frequency or speed setpointvalue 31 as an input to the current-frequency control component 24.Other embodiments are possible in which the rate limiter 30 is omitted,with the current-frequency component 24 directly receiving the desiredfrequency or speed signal 21 as a setpoint input. In the illustratedimplementation, the output signal or value from the rate limiter 30 is arate-limited frequency or speed setpoint signal or value 31 (e.g.,f_(RL)), and the rate limiter component 30 can be any suitable hardware,processor-executed software, processor-executed firmware, programmablelogic, analog circuitry, etc. which limits the rate of change of thereceived desired speed or frequency signal 21.

In one possible implementation, for instance, the rate limiter 30 limitsthe rate of change of the speed signal 21 such that the output signal 31is at a frequency which changes no faster than the maximum accelerationcapability of the motor 6. In operation, a step change in the receivedsignal 21 will be changed to a ramp signal 31, and thus the rate limiter30 prevents the subsequent current-frequency control component 24 fromdemanding an immediate change to high frequency current. Particularlywhen used with an output sine wave filter 16 and/or a transformer 18, animmediate change to high frequency current output may not cause themotor load 6 to rotate. Use of the rate limiter 30 in certainembodiments advantageously limits the rate of change of the frequencysetpoint down to the value where the motor load 6 can accelerate at thedesired rate.

The rate limited frequency or speed setpoint value 31 is provided as aninput to the current-frequency control component 24 as well as to anintegration system 40, 42 as described further below. Thecurrent-frequency (I-F) control component 24 receives the rate-limitedfrequency or speed setpoint 31, and generates a □□ axis current setpoint(i*_(δ)) 32 accordingly. As depicted in FIG. 2, the controller 20implements various components, for example, in processor-executedsoftware or firmware, and operates on certain variables in a synchronousδ, γ reference frame, with received feedback signals or values 28 andgenerated switching control signals 22 being reference to a stationary(e.g., a, b, c) reference frame. In this regard, the illustrated δ, γreference frame rotates at the same frequency as the conventional fieldcommutation control (D, Q) reference frame, but the position need not bethe same, with γ and δ somewhat analogous to “d” and “q”, but they arenot necessarily aligned (e.g., γ will likely be somewhere between the Daxis and the Q axis, and γ and δ are orthogonal to one another). It isalso understood that current regulation can be performed in otherreference frames.

As seen in FIG. 2, the current-frequency control component 24 provides acurrent setpoint output 32 based on the received (e.g., rate-limited)frequency or speed setpoint signal or value 31. In one possibleimplementation, the current-frequency control component 24 implements adual-range curve or function as illustrated, with the current-frequencyrelationship being a zero current value corresponding to a zerofrequency value (e.g., 0 Hz). As shown in FIG. 2, the current-frequencyrelationship implemented by the control component 24 includes a firstportion with increasing current values corresponding to a firstfrequency range from the zero frequency value to a cutoff frequencyvalue F_(CUT), as well as a second portion with a constant current value(e.g., I_(MAX)) corresponding to frequencies above the cutoff frequencyF_(CUT), where I_(MAX) can be the maximum rated output current of theinverter 14 in certain implementations, and the cutoff frequency F_(CUT)is preferably set to correspond to a very low operating frequency of themotor 6 (e.g., about 0.5-1.0 Hz in one implementation). Thecurrent-frequency control component 24 in certain embodiments can beimplemented using a lookup table or a parametric function. In thisregard, the current-frequency relationship advantageously avoidsproviding current to the transformer 18 and the motor 6 at zerofrequency, and includes the first ramped portion until the cutofffrequency, after which the maximum current is demanded, whereby thecontrol component 24 avoids sending DC to the transformer 18. The I-Fcontrol component 24 thus avoids sending DC to the transformer 18, andthe motor load 6 is typically operated at the maximum current I_(MAX),whereby the operation of the motor drive 10 is very different from theconventional voltage-frequency approach of prior sensorless drives.

The output (i*_(δ)) of the current-frequency controller 24 is the δ axiscurrent setpoint 32, which is provided to the PI control component 26.PI control is not a strict requirement of all embodiments of the presentdisclosure, wherein any suitable current regulating algorithm can beused to regulate the inverter output currents IA, IB and IC at analgorithm bandwidth that is less than the resonant frequency of the sinewave filter 16. In the illustrated embodiment, the PI controller 26operates according to a zero γ axis value 33 (i*_(γ)=0), although not astrict requirement of all implementations of the present disclosure. ThePI controller 26 can be any suitable implementation of well-knownproportional-integral control algorithms, but the control algorithm isbandwidth limited. Also, the control component 26 can be a PIDcontroller with the corresponding derivative gain (KD) set to zero. Theinventors have appreciated that limiting the bandwidth of the PIcontroller 26 avoids or mitigates large inrush current during power up,particularly where the drive 10 is providing output currents through asine wave filter 16 and/or transformer 18. In certain applications, forinstance, the sine wave filter 16 makes the inverter output particularlysusceptible to large inrush currents, and limiting the bandwidth of thePI controller 26 (or other current regulation control algorithmimplemented by the controller 20) to be well below the sine wave filterresonant frequency helps to mitigate or avoid high inrush currentlevels, particularly at power up.

The reduced bandwidth PI controller 26 also receives feedback from theoutput of the inverter 14 from on-board output current sensors 27. Theillustrated controller 20 includes a stationary-to-synchronous referenceframe converter component 44 (a,b,c→δ,γ) which provides δ and γ currentfeedback values 46 and 48 (i_(δ) and i_(γ)), converted from the sensedinverter output phase currents IA, IB and IC as inputs to the PIcontroller 26. The converter 44, moreover, performs the reference frameconversion according to a phase angle signal or value θ 43, which iscomputed in the illustrated embodiment based on the rate limitedfrequency or speed setpoint signal or value 31 as the integral of thefrequency ω (2Π*f_(RL)) via a multiplier component 40 generating thefrequency signal ω 41 and an integrator component 42 providing the phaseangle signal or value θ 43. The PI controller 26, moreover, provides δand γ axis voltage setpoint signal or value outputs V_(δ) and V_(γ) 35and 34, respectively, which are converted to the stationary referenceframe by converter 36 (δ,γ→a,b,c) using the phase angle signal or valueθ 43. The reference frame converter 36, in turn, provides a set of threestationary reference frame voltage setpoint signals or values 37 (V_(a),V_(b) and V_(c)) as inputs to a pulse width modulation (PWM) component38 that includes any suitable form of modulation, isolation, amplifiers,driver circuitry, etc. to generate the inverter switching controlsignals 22 using known techniques.

In certain embodiments, the bandwidth of the PI or other regulationcontrol algorithm implemented in the control component 26 is well belowthe resonant frequency of the associated sine wave filter 16. In thisregard, conventional servo and/or motor drive control algorithmsregulate current using a relatively high bandwidth, such as on the orderof 1 kHz. However, as mentioned above, such high control algorithmbandwidth may lead to instability or inability to properly control theoutput currents provided to the motor load 6, and/or lead to undesirablesaturation of the transformer 18 and excessive inrush current problems.In accordance with the present disclosure, the bandwidth of the PIcontroller 26 is preferably one or more orders of magnitude lower thanthe resonant frequency of the sine wave filter 16. For example, the PIcontroller bandwidth 26 may be on the order of about 20 Hz or 30 Hz foruse in association with sine wave filters 16 having a resonant frequencyof about 2 kHz to 6 kHz. This can be implemented, for example, bylimiting the proportional and integral gains (e.g., KP and KI) used inthe regulation algorithm of the PI controller 26. The output of the PIcontroller 26 in this regard may be implemented as the summation of theerror between the current setpoint values 32 and 33 and thecorresponding feedback values 46 and 48, multiplied by the proportionalconstant KP added to the integral of the error multiplied by theintegral constant KI. Thus in specific embodiments, the current loopregulation bandwidth is significantly below the resonant range of thefilter 16, and this can be implemented by suitable caps or limits on theKI and the KP values used in the PI algorithm 26 such that the resonantpoint of the entire closed loop is less than about 20 or 30 Hz incertain implementations. Thus, the current regulation algorithmimplemented by the controller 20 will not attempt to regulate currentabove about 30 Hz. The output 34, 35 of the PI controller 26 is thesynchronous reference frame voltage values 34 and 35, which are thentranslated into the three-phase stationary reference frame values 37that are used for pulse width modulating the inverter switches S1-S6.

Referring also to FIG. 3, a flow diagram is provided illustrating amethod 100 for controlling a power conversion system (e.g., the motordrive 10 above) to drive a load (e.g., motor 6) through a filter (e.g.,sine wave filter 16). Although the exemplary method 100 is depicted anddescribed in the form of a series of acts or events, it will beappreciated that the various methods of the disclosure are not limitedby the illustrated ordering of such acts or events except asspecifically set forth herein. In this regard, except as specificallyprovided hereinafter, some acts or events may occur in different orderand/or concurrently with other acts or events apart from thoseillustrated and described herein, and not all illustrated steps may berequired to implement a process or method in accordance with the presentdisclosure. The illustrated methods may be implemented in hardware,processor-executed software, or combinations thereof, in order toprovide sensorless motor control using bandwidth limited controlalgorithms as described herein, and various embodiments orimplementations include non-transitory computer readable mediums havingcomputer-executable instructions performing the illustrated anddescribed methods. For instance, the method 100 may be implemented usingone or more processors associated with the controller 20, by executinginstructions stored in an electronic memory operatively associated withthe controller 20.

The process 100 begins at 102 where an updated desired frequency orspeed value is received (e.g., signal or value f* 21 in FIGS. 1 and 2above). At 104, the desired value f* may optionally be rate limited toprovide a rate limited frequency or speed setpoint value (e.g., f_(RL)31 in FIG. 2). At 106 in FIG. 3, a current setpoint value is determined(□□ axis current setpoint i*_(δ) 32 in FIG. 2) at least partiallyaccording to the frequency or speed setpoint value 21, 31. At 108, oneor more AC output current feedback signals or values of the powerconverter are sampled (e.g., IA, IB and IC sampled using sensors 27 inFIG. 1 above). At 110, the AC output current(s) is/are regulatedaccording to the current setpoint value 32 and the feedback signal(s) orvalue(s) 28 using a control algorithm having a bandwidth below theresonant frequency of the filter 16. For instance, the AC outputcurrents are regulated in the above described motor drive 10 using aproportional-integral control algorithm (e.g., via PI controller 26)having a bandwidth that is less than the resonant frequency of the sinewave filter 16. The process 100 in FIG. 3 then returns to receiveanother updated desired frequency or speed value 21 at 102 and continuesas described above.

The above techniques and apparatus thus advantageously facilitatesensorless control of induction or permanent magnet motor loads 6 inapplications such as those described supra in which sine wave filters 16and/or transformers 18 are provided to accommodate long cable lengths 8without the shortcomings associated with conventional voltage-frequencycontrol schemes. In addition, the disclosed concepts can be employedwithout any additional hardware, and may be implemented largely inprocessor-executed software of a motor drive controller 20. Thesetechniques, moreover, may be employed to reduce or avoid filter inrushcurrent, transformer saturation, and uncontrolled motor drive outputcurrent oscillation associated with conventional approaches, whileavoiding extra cost and/or system complexity associated with provisionof sensors at the load 6. In addition, the techniques also allow thesystem design flexibility of employing transformers 18 to accommodatelong cable runs while reducing or avoiding I²R losses and allowing theuse of smaller cabling 8, thereby providing viable solutions tosubmersible pump applications and other installations where thetransformer 18 can accommodate use of low-voltage motor drives 10 formedium or higher voltage motors 6, including permanent magnet motors.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described components (assemblies, devices,systems, circuits, and the like), the terms (including a reference to a“means”) used to describe such components are intended to correspond,unless otherwise indicated, to any component, such as hardware,processor-executed software, or combinations thereof, which performs thespecified function of the described component (i.e., that isfunctionally equivalent), even though not structurally equivalent to thedisclosed structure which performs the function in the illustratedimplementations of the disclosure. In addition, although a particularfeature of the disclosure may have been disclosed with respect to onlyone of several implementations, such feature may be combined with one ormore other features of the other implementations as may be desired andadvantageous for any given or particular application. Also, to theextent that the terms “including”, “includes”, “having”, “has”, “with”,or variants thereof are used in the detailed description and/or in theclaims, such terms are intended to be inclusive in a manner similar tothe term “comprising”.

The following is claimed:
 1. A power conversion system, comprising: aninverter operative to provide AC output power to drive a load; and acontroller operative to regulate at least one inverter output currentaccording to a frequency or speed setpoint value via a control algorithmhaving a current-frequency relationship with a zero current valuecorresponding to a zero frequency value, and a bandwidth below aresonant frequency of a filter coupled between the inverter and theload.
 2. The power conversion system of claim 1, wherein the controllercomprises: a current-frequency control component providing a currentsetpoint value according to the frequency or speed setpoint value andthe current-frequency relationship with the zero current valuecorresponding to the zero frequency value; and a current controlregulator component implementing the control algorithm to regulate theinverter output current according to the current setpoint value.
 3. Thepower conversion system of claim 2, wherein the current controlregulator component implements the control algorithm to regulate theinverter output currents according to the current setpoint value and atleast one feedback signal or value representing the inverter outputcurrent.
 4. The power conversion system of claim 3, wherein thecontroller comprises a rate limiter component limiting a rate of changeof a received desired frequency or speed value to provide the frequencyor speed setpoint value.
 5. The power conversion system of claim 2,wherein the controller comprises a rate limiter component limiting arate of change of a received desired frequency or speed value to providethe frequency or speed setpoint value.
 6. The power conversion system ofclaim 2, wherein the controller includes a proportional-integralcontroller to regulate the inverter output currents, theproportional-integral controller having a bandwidth below the resonantfrequency of the filter coupled between the inverter and the load. 7.The power conversion system of claim 6, wherein the controller regulatesthe inverter output current according to a current setpoint valuederived from the frequency or speed setpoint value and according to atleast one feedback signal or value representing the inverter outputcurrent.
 8. The power conversion system of claim 6, wherein thecontroller comprises a rate limiter component limiting a rate of changeof a received desired frequency or speed value to provide the frequencyor speed setpoint value.
 9. The power conversion system of claim 2,wherein the current control regulator component is aproportional-integral controller having a bandwidth below the resonantfrequency of the filter coupled between the inverter and the load. 10.The power conversion system of claim 9, wherein the controller comprisesa rate limiter component limiting a rate of change of a received desiredfrequency or speed value to provide the frequency or speed setpointvalue.
 11. The power conversion system of claim 9, wherein the currentcontrol regulator component implements the control algorithm to regulatethe inverter output current according to the current setpoint value andat least one feedback signal or value representing the inverter outputcurrent.
 12. The power conversion system of claim 11, wherein thecontroller comprises a rate limiter component limiting a rate of changeof a received desired frequency or speed value to provide the frequencyor speed setpoint value.
 13. The power conversion system of claim 1,wherein the controller regulates the inverter output current accordingto a current setpoint value derived from the frequency or speed setpointvalue and according to at least one feedback signal or valuerepresenting the inverter output currents.
 14. The power conversionsystem of claim 1, wherein the controller comprises a rate limitercomponent limiting a rate of change of a received desired frequency orspeed value to provide the frequency or speed setpoint value.
 15. Amethod for controlling a power conversion system driving a load througha filter, the method comprising: determining a current setpoint valueaccording to a frequency or speed setpoint value; sampling at least oneAC output current feedback signal or value of the power conversionsystem; and regulating the at least one AC output current according tothe current setpoint value and the at least one AC output currentfeedback signal or value using a control algorithm having acurrent-frequency relationship with a zero current value correspondingto a zero frequency value, and a bandwidth below a resonant frequency ofthe filter.
 16. The method of claim 15, comprising limiting a rate ofchange of a desired frequency or speed value to determine the frequencyor speed setpoint value.
 17. The method of claim 15, wherein the currentsetpoint value is determined according to a current-frequencyrelationship with a zero current value corresponding to a zero frequencyvalue.
 18. The method of claim 17, wherein the current-frequencyrelationship includes a first portion with increasing current valuescorresponding to a first frequency range from the zero frequency valueto a cutoff frequency value, and a second portion with a constantcurrent value (I_(MAX)) corresponding to frequencies higher than thecutoff frequency value.
 19. The method of claim 15, wherein the at leastone AC output current is regulated using a proportional-integral controlalgorithm having a bandwidth below the resonant frequency of the filter.20. A non-transitory computer readable medium with computer executableinstructions for controlling a power conversion system driving a loadthrough a filter, the computer readable medium comprising computerexecutable instructions for causing a processor when executed to:determine a current setpoint value according to a frequency or speedsetpoint value; sample at least one AC output current feedback signal orvalue of the power conversion system; and regulate the at least one ACoutput current according to the current setpoint value and the at leastone AC output current feedback signal or value using a control algorithmhaving a current-frequency relationship with a zero current valuecorresponding to a zero frequency value, and a bandwidth below aresonant frequency of the filter.